This invention relates to the polymerization of water-miscible monomers in aqueous solution.
Most water-soluble polymers, particularly polymers of acrylamide, acrylic acid and their water-miscible derivatives have been employed commercially as additives in the manufacture of paper products, as water purification coagulants, as dispersing agents and as treating agents in a wide variety of other applications.
Of the various methods employed to polymerize water-miscible monomers, the aqueous solution polymerization method is the most commonly employed because it is less expensive and it readily provides water-soluble polymers having the high molecular weight that is essential for many applications. In such a method, the concentrations of monomer and resulting polymer in aqueous solution are maintained as high as possible in order to maximize the capacity of the polymerization apparatus and to minimize the amount of water that is subsequently removed from the resulting polymer product. Unfortunately, however, concentrated aqueous solutions, i.e., those in excess of 2 to 3 weight percent of the resulting water-soluble polymer, are extremely viscous. Thus, in highly concentrated solutions of the water-soluble polymer, it is virtually impossible to provide adequate heat transfer by stirring so that a uniform polymerization temperature is maintained throughout the polymerization vessel. In addition, it is difficult to remove the viscous polymer solution from the vessel once polymerization is complete.
To solve the aforementioned problems resulting from the high viscosities of relatively dilute aqueous solutions of such water-soluble polymers, it has been a practice to suspend or emulsify the aqueous solution of water-miscible monomer in a water-immiscible organic liquid such as a liquid hydrocarbon; then, the monomer in suspension or emulsion is polymerized to form a relatively unstable suspension or a relatively stable emulsion of the desired water-soluble polymer. Methods of practicing such suspension or emulsion polymerization are described in U.S. Pat. Nos. 2,982,749 and 3,284,393, respectively. In following the aforementioned suspension or emulsion polymerization methods, the concentration of monomer, and resulting polymer, in the suspension or emulsion may be increased to about 30-40 weight percent based on the weight of the suspension or emulsion. In order to achieve greater reduction in the quantity of water to be removed as well as to reduce the quantity of expensive hydrocarbon liquid, it has been desirable to further increase concentration of the monomer and resulting polymer in the suspension or emulsion. Unfortunately, attempts to increase the concentration of monomer, and resulting polymer, significantly above the 30-40 weight percent level normally used have generally been unsuccessful due to premature coagulation of the dispersed polymer and/or loss of desired polymer properties. For example, it is often very difficult to produce highly concentrated emulsions or suspensions of the polymer wherein the polymer has a relatively high molecular weight but no insolubles (gels). Also, it has often been difficult to practice such methods in a continuous manner.
In view of the aforementioned deficiencies of the prior art methods, it is highly desirable to provide a continuous process for polymerizing water-miscible monomers in relatively concentrated aqueous solutions to form water-soluble polymers that have uniformly high molecular weight and are relatively free of water-insoluble gels.